1. Field of the Invention
The present invention relates generally to electrical apparatus and, more particularly, to shields for protecting electrical components of electrical apparatus. The invention further relates to printed circuit boards employing shields, and to electrical apparatus employing printed circuit boards having a shield.
2. Background Information
Electrical apparatus such as, for example, a molded case circuit breaker having an electronic trip unit, include a plurality of electrical components which are susceptible to damage and, therefore, must be protected.
Specifically, the electronic trip unit of the molded case circuit breaker typically includes a printed circuit board (PCB), with a plurality of electrical components being coupled to the PCB. The PCB facilitates tripping the circuit breaker in response to a trip condition (e.g., without limitation, an overcurrent condition, an overload condition, or a relatively high level short circuit or fault condition). A high current fault interruption, for example, typically results in a substantial arc in the arc chamber of the circuit breaker. The arc forms rapidly expanding gases, and may generate flames and flying debris comprised of molten metal particles and fragments of various circuit breaker components which are expelled throughout the breaker. As a consequence, the electrical components of the PCB may be susceptible to malfunction due, for example, to physical harm caused by the arc, or to a short circuit or open circuit caused by exposure to the byproducts of the arc.
PCBs employ a conformal coating (e.g., for example and without limitation, Humiseal® available from Chase Specialty Coatings which has a place of business in Pittsburgh, Pa.) primarily to protect against humidity and undesirable affects associated therewith. However, such coatings provide only relatively minor protection during high current fault interruption. As such, in order to shield the electrical components from flying debris, one known prior proposal has been to surround and enclose the PCB in a thermoplastic clamshell-type molded casing or shield. However, holes in the clamshell, such as thru holes for receiving components and openings between the halves of the clamshell, allow gas-carried carbon to enter and accumulate. The gas-carried carbon can become trapped proximate sensitive electrical components resulting in an undesirable condition. Specifically, carbon film deposits which coat the PCB as hot gases escape, cause high leakage current which is a leading cause of PCB malfunction and/or failure. Moreover, the trapped carbon can cause short circuits between adjacent electrical components and/or reduce normal conductivity that undesirably inhibits the flow of electrical current. In addition, assembly of the thermoplastic clamshell is a labor and cost intensive process which involves placing the clamshell halves around the PCB and then securing one half to the other.
There is, therefore, room for improvement in shields for electrical apparatus.
There is also room for improvement in printed circuit boards, and in electrical apparatus, such as electronic molded case circuit breakers, which employ printed circuit boards.